In many applications air is supplied to the exhaust system of an automotive engine to support oxidation of undesirable exhaust gas constituents. In some such applications, it may be desirable to limit the rate of air flow to the exhaust system to a proportion of the rate of exhaust flow through the exhaust system. U.S. Pat. No. 3,919,842 issued Nov. 18, 1975 in the name of R. A. Bolton shows one example of a valve which limits the rate of air flow to a proportion of the rate of exhaust flow.